This invention relates to a method for making tea products, specifically extracts and powders, that are soluble in both hot and cold water.
Leaf tea may be prepared as green leaf tea or black leaf tea. The method of preparing such teas is well-known to those skilled in the art. Generally, to prepare black leaf tea, fresh green leaves are withered (subjected to mild drying), comminuted, fermented (in which process enzymes in the tea leaf use atmospheric oxygen to oxidise various substrates to produce brown-coloured products) and then fired (to dry the tea leaves). Green leaf tea is not exposed to the fermentation and firing processes. Partial fermentation may be used to produce intermediate-type teas known as xe2x80x9coolongxe2x80x9d tea.
When hot aqueous infusions of black leaf tea are prepared, it is found that the infusion comprises substances which are insoluble in cold water, which substances therefore tend to precipitate as the infusion cools. These cold water insoluble substances comprise tannin complexes (known as tea xe2x80x9ccreamxe2x80x9d) and typically comprise from 15-35% of the total tea solids in the infusion.
Black leaf tea infusions may be used to produce xe2x80x9cinstantxe2x80x9d teas and other products which are preferably soluble in cold water. For this reason, it is known to separate the insoluble tea cream from the xe2x80x9cdecreamedxe2x80x9d fraction (which is the term given to the cold water soluble materials after removal of the cold water insoluble cream). This is typically accomplished by centrifugation of the chilled (3-10xc2x0 C.) extract. The insoluble cream fraction represents a significant proportion of the tea solids in the infusion. Accordingly, to prevent the cream fraction (which contains desirable flavour components) going to waste, it is known to treat the cream fraction, in one of a number of ways, so as to render it soluble in cold water and then to recombine the solubilised cream with the decreamed fraction. Various treatments of the cream fraction of tea infusions are described, for example, in GB 1,311,255, GB 1,461,726, U.S. Pat. Nos. 3,787,590, 4,156,024 and 5,827,560.
In contrast, less is known about treatment of whole tea infusions, without prior separation of the cream and decreamed fractions. Whole tea infusions differ substantially in terms of chemical composition relative to the separated cream portion.
United States patent specification U.S. Pat. No. 4,680,193 (Nestlxc3xa9) discloses a process in which a whole black tea infusion, containing cold water insoluble substances, is mixed with catechins in order to solubilise the insoluble material.
European patent specification EP 0,067,351 (Nestlxc3xa9) describes a process for making a powdered tea extract. The process involves making two aqueous extractions of black tea leaves: one at ambient temperature using an aqueous solution of a carboxylic acid and/or salt thereof; and a second extract using water at elevated temperature. Both extractions are performed at ambient pressure.
However, contrary to the prior art above the present inventors achieve oxidation of the whole tea solids without any pH modification or catalyst addition but solely through a combination of high pressure processing and an optimised oxygen transfer rate. That is significant as the incomplete removal of pH modifying substances and catalysts cause contamination.
It is an object of the present invention to provide a method for making a cold water soluble black tea product from a whole black tea infusion.
In a first aspect the invention can be said in broad terms to relate to a method for making a cold water soluble black tea extract, the method comprising the steps of (a) extracting tea solids from black tea leaves using an extraction liquid to provide a whole tea extract; (b) oxidising said whole tea extract under superatmospheric pressure and at a temperature above 60xc2x0 C. to provide a cold water soluble liquor; (c) cooling said cold water soluble liquor to precipitate any residual cold water insoluble material; and (d) separating said residual cold water insoluble material from said cold water soluble liquor to give the cold water soluble black tea extract.
In a second aspect the invention can be said in broad terms to relate to a method for making a cold water soluble black tea powder, the method comprising the steps of (a) extracting tea solids from black tea leaves using an extraction liquid to provide a whole tea extract; (b) oxidising said whole tea extract under superatmospheric pressure and at a temperature above 60xc2x0 C. to provide a cold water soluble liquor; (c) cooling said cold water soluble liquor to precipitate any residual cold water insoluble material; (d) separating said residual cold water insoluble material from said cold water soluble liquor to give the cold water soluble black tea extract; and (e) drying the cold water soluble black tea extract to form the cold water soluble black tea powder.
Preferably the oxygen transfer rate is between 8 and 50 hrxe2x88x921, more preferably between 15 and 35 hrxe2x88x921. The oxygen can be provided as a component of air.
Beverages made from these tea powders have good clarity and favourable flavour and colour.
xe2x80x9cTeaxe2x80x9d for the purposes of the present invention means leaf material from Camellia sinensis var. sinensis or Camellia sinensis var. assamica. xe2x80x9cTeaxe2x80x9d is also intended to include the product of blending two or more of any of these teas.
For the avoidance of doubt the word xe2x80x9ccomprisingxe2x80x9d is intended to mean including but not necessarily xe2x80x9cconsisting ofxe2x80x9d or xe2x80x9ccomposed ofxe2x80x9d. In other words the listed steps or options need not be exhaustive.
Except in the operating and comparative examples, or where otherwise explicitly indicated, all numbers in this description indicating amounts or concentrations of material ought to be understood as modified by the word xe2x80x9caboutxe2x80x9d.
The invention relates to a method for making a cold water soluble black tea extract or powder. The method involves extracting tea solids from black tea leaves using an extraction liquid to provide a whole tea extract and oxidising the whole tea extract, as opposed to solely the insoluble cream fraction or the soluble decreamed fraction, under superatmospheric pressure and at a temperature above 60xc2x0 C. This gives a cold water soluble liquor. Further method steps involve cooling the cold water soluble liquor to precipitate any residual cold water insoluble material, and separating the residual cold water insoluble material from the cold water soluble liquor to give the cold water soluble black tea extract.
The inventors have found that the temperature and pressure are preferably those at which distilled water would have at equilibrium a maximum capacity for dissolved oxygen of at least 0.5 grams per liter. It is also preferred the oxygen transfer rate is such that the tea solids are oxidised in the absence of a catalyst or any pH modification step.
It will be appreciated by those skilled in the art that, at the temperatures indicated above, the cold water insoluble tea solids will be generally soluble, such that where the method is applied to an aqueous extract prepared from black tea leaves, the aqueous mixture will be substantially a solution. The aqueous mixture will conveniently be an aqueous whole black tea extract, comprising cold water soluble tea materials as well as the insoluble materials.
The maximum obtainable concentration of dissolved oxygen in distilled water under given conditions of temperature and pressure can be readily determined by reference to standard texts. Thus Perry""s Chemical Engineering Handbook (Perry and Green 1984, Sixth Edition p.3 103, McGraw Hill) gives the values for maximum solubility of oxygen in water at elevated pressure for temperatures up to 100xc2x0 C. For temperatures in excess of 100xc2x0 C., reference may be made to the paper by Pray et al., (1952 Industrial and Engineering Chemistry 44, 1146-1151). The maximum dissolved oxygen concentration obtainable at equilibrium under the same conditions in aqueous mixtures comprising cold water insoluble tea solids (such as are the subject of the process of the present invention) may vary somewhat from those values obtainable in distilled water. In particular the presence of other solutes in the aqueous phase, competing with oxygen molecules for hydration by water molecules, will tend to decrease the solubility of oxygen in the aqueous phase. However, this is unlikely to cause a large reduction in oxygen solubility in the conditions of interest. The actual concentration of dissolved oxygen in the aqueous mixture cannot readily be determinedxe2x80x94under the conditions of the process, standard methods of determining oxygen concentration (e.g. by the use of an oxygen electrode) are not feasible.
The conditions employed in the method defined above are more extreme than those conventionally applied in the prior art and result in a far greater maximum obtainable dissolved oxygen concentration for the aqueous mixture than hitherto. In general it would be preferred to avoid such conditions because of the energy costs in achieving the same.
However, the present inventors have found that such processing when applied to black tea extracts, largely solubilises the cold water insoluble materials present in the mixture, and has a highly desirable effect on the colour of mixture, which becomes much darker (i.e. less luminous) and more highly red coloured, than is generally achievable in conventional processes.
The preferred oxidant is oxygen. Use of high partial pressures of oxygen serves to increase the maximum capacity of the aqueous mixture for dissolved oxygen. Preferably conditions are such as to create a maximum capacity for dissolved oxygen (in distilled water) at equilibrium in the range of 0.5 to 5 grams per liter, more preferably 0.5 to 1.5 grams per liter, and most preferably 0.7 to 1.0 grams per liter.
Preferably conditions are arranged (e.g. by the use of high partial pressures of oxygen and by the use of agitation) such that the actual concentration of dissolved oxygen in the aqueous mixture approaches the maximum obtainable at equilibrium under the selected conditions. However, it is quite possible that the system never attains equilibrium (e.g. because dissolved oxygen is consumed in oxidation reactions), such that the maximum obtainable equilibrium concentration of dissolved oxygen in the aqueous mixture is not reached.
Those skilled in the art will appreciate that other oxygen-containing or generating substances may be used to give an equivalent oxygen solubility in the aqueous mixture. For example, a higher partial pressure of air or oxygen-enriched air may be used, or (less preferably) aqueous solutions of hydrogen peroxide may be added. Alternatively ozone, or other oxidising gas, may be used so as to give an xe2x80x9coxidising powerxe2x80x9d in the aqueous mixture equivalent to that generated by a maximum oxygen solubility of at least 0.5 grams per liter.
It will be apparent from the foregoing, and those skilled in the art would appreciate, that increased temperature in a closed reaction system will increase pressure, and so tend to increase the amount of oxygen dissolving in the aqueous mixture. Under certain circumstances, it may be preferred to use an xe2x80x9copenxe2x80x9d system, whereby the concentration of a gaseous oxidising agent is held constant, whilst being passed through the reaction vessel at a given flow rate. Alternatively, the gaseous oxidising agent may be advantageously introduced in pulses.
The reaction may be performed as a batch process (where the reaction vessel may be, for example, a stirred tank) or may be a continuous process (performed, for example, in a stirred tank or a conduit, such as a pipe).
The method of the invention may successfully be performed on aqueous mixtures comprising suspensions of cold water insoluble tea solids in the range 0.3-20.0% (w/v). Conveniently a concentration in the range 3-10% (w/v) may be selected.
In relation to the processing of cold water insoluble black tea solids, in the examples that follow, a 3% (w/v) suspension of tea solids was prepared in deionised water, starting from a freeze-dried powder prepared from an aqueous tea extract. This arrangement allowed for optimum reproducibility of experimental conditions and had the advantage of simplicity. In practice, on an industrial scale, it is envisaged that the aqueous mixture used in the process of the invention will be an aqueous tea extract, without having gone through an initial freeze-drying stage. The aqueous extract may conveniently be concentrated prior to processing according to the method of the invention.
Aqueous black tea extracts are naturally acidic, typically having a pH in the range 4.0-5.0. In conventional processes, efficient solubilisation of cold water insoluble materials (performed without application of high pressures) requires the addition of a catalyst and/or the addition of a strong base, such as sodium hydroxide, to increase the pH to 8-11, to allow oxidation of the cold water insoluble materials. As oxidation proceeds the pH falls. However, the fall in pH is usually insufficient to return the mixture to its native pH range, such that acid is required to be added after the oxidation stage in order to obtain a more natural product.
The present inventors have found that use of an oxidising agent at high pressure and temperature allows a considerable degree of solubilisation to occur at lower pH than is generally used in conventional processes, avoiding the requirement for the addition of a strong base to achieve alkaline conditions. For example, solubilisation of black tea cold water insoluble solids can be achieved using a pH of 5.0 to 7.0, more particularly 5.5 to 6.5. This modestly increased pH can satisfactorily be achieved by the addition to the aqueous mixture of a salt of a weak acid, such as trisodium citrate. Again, as the pH falls during oxidation/solubilisation of the cold water insoluble materials, the final pH of the aqueous solution is very close to the pH of the native material, giving a naturally acidic product.
If desired, pressures and temperatures towards the lower end of the ranges defined above can be selected, together with a correspondingly more alkaline pH, without loss of solubilising effect. Mildly alkaline pHs (e.g. pH 8.0) may be achieved by the addition of a strong base, such as NaOH, to the aqueous mixture. Conversely, higher pressures and temperatures can be used with native pHs. There would not appear to be any theoretical maximum pressure, although clearly there are practical constraints to consider (e.g. strength of reaction vessel etc.).
The preferred temperature at which the process is performed varies slightly depending on the substrate. The preferred temperature for processing either black tea solids is above 100xc2x0 C., preferably 100 to 140xc2x0 C., more particularly 110 to 120xc2x0 C. The preferred oxygen partial pressure is 0.5-2.0 MPa.
The time taken to complete the reaction will of course depend in part on the reaction conditions used and the degree of solubilisation required. Generally, more extreme conditions (higher temperature/pressure) will result in quicker attainment of a desired end-point. Typically, the reaction will take between 10 minutes and 1 hour, more normally 10 to 30 minutes. The reaction time may be shortened by the incorporation of other oxidising agents (e.g. ozone, H2O2) into the aqueous mixture, either in a single batch or incrementally.
If desired, any residual cold water insoluble material may be removed at the end of the process. Typically this will be performed by cooling the mixture to precipitate the cold water insoluble material, followed by centrifugation (which step is generally known as xe2x80x9cpolishingxe2x80x9d). Finally, the resulting solution may optionally be concentrated and dried, typically by spray drying or freeze-drying, to give a cold water soluble instant tea powder. Such a powder can be used to make ready to drink tea products.